The present invention relates to an audio device such as a component car stereo to be mounted on an automobile and, more particularly, to an audio device of the multi-amplifier system in which an output of a single preamplifier is connected to inputs of a plurality of main amplifiers.
In the conventional car mounted audio device, a car battery is used for a power source thereof. Therefore, when the multi-amplifier system where a single preamplifier drives a pair of power amplifiers is employed, input side grounded points of the main amplifiers are commonly connected in the preamplifier and form a grounding loop as shown in FIG. 1. Therefore, noise component overlapped on the power source due to the loop, i.e., noises and ripple components of signal generated by the car, will be supplied to the input terminals of the power amplifiers. As a result, the noise component is amplified by the main amplifiers. In FIG. 1, an output signal e.sub.i of the preamplifier A which is included in a musical source such as a cassette deck or radio tuner mounted on the car is supplied to input terminals I.sub.1 and I.sub.2 of a pair of the power amplifiers B.sub.1 and B.sub.2 which are provided separately from the musical source. Output terminals O.sub.1 and O.sub.2 of the respective power amplifiers B.sub.1 and B.sub.2 are connected to a pair of loud speakers SP.sub.1 and SP.sub.2 to drive the latter. The above circuit construction forms the so-called multi-system. In this case, when the physical distance between preamplifier A and the power amplifier B.sub.2, for example, in considerable, there is provided an impendance in a path extending between a grounding terminal E.sub.A of the preamplifier and a grounding terminal E.sub.2 of the power amplifier B.sub.2, through which currents caused by noises of an alternator, an ignition system and a horn of the car will flow. As a result, an equivalent noise signal e.sub.n is apparently inserted in series with the output signal e.sub.i of the preamplifier A between the signal output terminal O.sub.A of the preamplifier A and the input terminal I.sub.2 of the power amplifier B.sub.2, which is amplified by the power amplifiers to drive the loud speakers.
Furthermore, since a portion of main currents of the power amplifiers will flow through the grounding circuit, a ripple voltage is also inserted in series between the output terminal O.sub.A of the preamplifier A and the input terminal I.sub.2 of the power amplifier B.sub.2, causing the Klirr factor to be degraded.
In order to overcome these problems, it has been proposed to insert a line transformer between the preamplifier A and the power amplifier B.sub.2 to cut the grounding loop circuit. However, the line transformer is generally heavy and bulkey and the line transformer having good high frequency characteristics is very expensive. The number of the line transformers must be increased if two or more power amplifiers are to be used.
In order to resolve the problems inherent to the use of the line transformer, a circuit shown in FIG. 2 has been proposed. In FIG. 2, the output signal e.sub.i of the signal preamplifier A is applied through a pair of variable feder resistors R.sub.1 and R.sub.2 to differential inputs of a pair of differential amplifiers D.sub.1 and D.sub.2 which function to cut the grounding circuit, respectively. That is, the signal output terminal O.sub.A of the preamplifier A is connected through the variable feder resistors R.sub.1 and R.sub.2 to forward phase input terminals of the differential amplifiers D.sub.1 and D.sub.2, respectively. The output terminals of the differential amplifiers are connected to the input terminals I.sub.1 and I.sub.2 of the power amplifiers B.sub.1 and B.sub.2, respectively, and the grounding terminal E.sub.A of the preamplifier A is connected to reverse phase input terminals of the differential amplifiers D.sub.1 and D.sub.2. The output terminals of the differential amplifiers are connected to the input terminals I.sub.1 and I.sub.2 of the power amplifiers B.sub.1 and B.sub.2. Grounding terminals E.sub.D1 and E.sub.D2 (which are common) of the differential amplifiers are connected to the grounding terminals E.sub.1 and E.sub.2 of the power amplifiers B.sub.1 and B.sub.2, respectively. Other components and connections therebetween in FIG. 2 are the same as those shown in FIG. 1. This circuit construction is very effective in the sence of removing the aforementioned problem.
In the circuit shown in FIG. 2, the noise voltage e.sub.n produced between the terminals E.sub.A and E.sub.2 is added directly between the reverse input terminal of the differential amplifier D.sub.2 and the grounding terminal (common terminal) E.sub.D2 of the same. On the other hand, to the non-reverse input terminal thereof, a signal e.sub.i +e.sub.n where e.sub.n is the noise voltage and e.sub.i ' is a feder-controlled voltage of the output signal voltage e.sub.i of the preamplifier A is applied. Since, in this case, the signals e.sub.n and e.sub.i '+e.sub.n are in phase, a signal K(e.sub.i '+e.sub.n -e.sub.n)=K.e.sub.i ' appears at the output of the differential amplifier D.sub.2, where K is an amplification factor of the differential amplifier D.sub.2.
Therefore, the noise voltage e.sub.n is completely removed by the differential amplifier D.sub.2. This is also applicable to the other differential amplifier D.sub.1.
Therefore, a signal corresponding only to the output signal e.sub.i of the preamplifier A is fed to the power amplifiers B.sub.1 and B.sub.2 and it is possible to drive the loud speakers SP.sub.1 and SP.sub.2 connected as a load, without noise.
However, there are cases where only one of the power amplifiers B.sub.1 and B.sub.2 is to be used according to demand of a user. In such case, the output of one of the differential amplifiers, say D.sub.2, becomes in an open state and thus the grounding terminal E.sub.D2 thereof becomes in the floating state.
With the circuit construction in which the car battery voltage V is directly applied to the positive power supply line of the differential amplifier D.sub.2, the floating condition of the grounding terminal E.sub.D2 may provide sources of undesirable noises affecting the whole circuit system through loops passing from the power supply line through internal circuit elements, the differential input thereof and the input terminal of the differential amplifier D.sub.2.